The invention relates to a current feeding device for electrical apparatus with conductors which are cooled to a low temperature in general and more particularly to a device of this nature utilizing a normal conductor which is cooled by a cold gas and consists of metal netting.
Current feeding devices are required for feeding current into electrical apparatus with conductors cooled to a low temperature. Electric current is fed through such devices to the deep-cooled conductors from a current supply which is at a higher temperature level, e.g., room temperature. Examples of such electrical apparatus are superconducting coils, cables or machines, the conductors of which contain superconductive material which is cooled to a temperature level below what is known as its transition temperature by means of a cryogenic medium e.g., liquid helium. Since this transition temperature of the known superconductive materials is far below room temperature, conductors of electrically normally conducting material such as copper or aluminum are used to bridge the respective temperature differences in the current feeding devices. These normally conducting conductors are then connected to the superconductors of the electrical apparatus at a point which is also kept at a temperature level below the transition temperature of the superconductor material. The normal conductors can be cooled from their portion at room temperature up to this junction gradually or in steps.
One known current feeding device of this nature is described in "Rev.Sci. Instr.", vol. 38, no. 12, December 1967, pages 1776 to 1779. Laminations, wires or netting are used as the normal conducting conductors of this current feeding device. Through these normal conductors, heat is supplied to a helium bath. Part of the liquid helium therefore evaporates and rises as cold helium gas through the conductor laminations or the conductor netting and thus removes the Joule heat produced as well as the heat flowing in from the outside. In this process, the helium gas is warmed up to about room temperature. At the upper junction point of the normal cnductor, which is coupled to an external current supply, the helium gas is collected and can be returned to refrigeration equipment for reliquefication.
If a constant temperature difference is to be maintained between an electric power supply and equipment with deep-cooled conductors, particularly superconductors, a problem therefore arises due to an undesireable heat exchange between the current supply and the deep-cooled conductors taking place through heat conduction through the normal conductors of the current feeding device. If, for instance, a superconductor is connected to a power supply which is at room temperature, considerable amounts of heat also flow into the superconductor or the cryogenic medium cooling it because of the finite heat conductivity of the normal conductor material of the current feeding device i.e. the normal conductor is not only a good electrical conductor but also a good heat conductor. The coolant losses connected with such heat transfer must be kept to a minimum. The requirement of minimizing the flow of heat into the cryogenic medium for the deep-cooled conductors thus exists.
To meet this requirement, an optimum geometric form of the normal conductors can be determined. The metal of the normal conductors must have high electric conductivity; however, its thermal conductivity should be small at the same time. In addition, the cross section must be both large enough that the Joule heat generated is relatively small and also must be small enough that the heat inflow is limited. Furthermore, the surface of the normal conductors, which is cooled by a cold gas, must be as large as possible so that an effective heat exchange with the cold gas is ensured.
A current feeding device in which these requirements are taken into account is described in U.S. Pat. No. 3,610,809. As the normal conductor of this known current feeding device, a wire gauze or a metal braid is provided, for instance. The conductor cross section is reduced in a central part of the current feeding device between its warm and cold end.
If the cross section is changed by appropriate cutting in such a net-like normal conductor the wires of the net which run in the axial direction of the current feeding device are interrupted and the current carried in them must pass to the wires that run in the transversal direction. Braided wire nets, however, only have touch contact between these intersecting wires. Thus, an additional ohmic transition resistance occurs at these cross points. No variations of the conductor cross section can therefore be accomplished with these braided nets without incurring additional ohmic losses and, thereby, also additional coolant losses.
To reduce these transition resistances, the braided metal nets can be soldered at the cross points between their transversally and longitudinally disposed wires (see U.S. Pat. 3,715,452). However, the method of doing this is relatively expensive.
In view of these difficulties, it is therefore an object of the invention to create a current feeding device which can be simply produced. Further, it is a particular object to produce such a device using conventional, braided metal netting without the occurrence of the difficulties mentioned above.